1. Field of the Invention
This invention relates to a charging member, an electrophotographic apparatus and a process cartridge.
2. Description of the Related Art
An elastic-material layer of a charging roller used in a contact charging system usually has an electrical conductivity of approximately from 1×103 to 1×107 Ω·cm in volume resistivity. In order to obtain an elastic-material layer having such an electrical conductivity, an electro-conductivity type conductive rubber composition compounded with conductive particles such as carbon black may be used to form the elastic-material layer. This is disclosed in Japanese Patent Application Laid-open No. H09-090714.
However, the elastic-material layer thus formed has, as disclosed in Japanese Patent Application Laid-open No. H09-090714, a problem that its electrical resistance depends so strongly on the state of dispersion of the conductive particles as to tend to cause resistance non-uniformity in the charging roller. Also, in the charging member having such an elastic-material layer, the conductive particles in the elastic-material layer may faster come to aggregate because of continuous application of direct-current voltage, to make the elastic-material layer change gradually in its electrical resistance.
Japanese Patent Application Laid-open No. 2007-292298 discloses that a bead mill may be used when the conductive particles such as carbon black are dispersed in a material for forming a conductive coat layer of the charging member and this enables achievement of the resolution of the problem that the charging member layer may change in electrical resistance with its service.